1. Field of the Invention
The present invention is generally in the field of high voltage circuits. More particularly, the invention is in the field of high voltage rectifier and switching circuits.
2. Background Art
A depletion-mode transistor can be connected in cascode with an enhancement-mode transistor to form an enhancement-mode switching circuit. The enhancement-mode switching circuit can have a high voltage, such as 600 volts, connected to a drain of the depletion-mode transistor and a low voltage, such as −10 volts, present at the gate of the depletion-mode transistor and at the source of the enhancement-mode transistor. Using such a configuration, the gate of the enhancement-mode transistor can be a control terminal of the enhancement-mode switching circuit, while the blocking voltage of the depletion-mode transistor can provide the blocking capability of the switching circuit.
The depletion-mode transistor can be, for example, a high electron mobility transistor (HEMT), such as a III-Nitride HEMT, which can support high current and high blocking capability, but it may not be desirable to form as an III-Nitride enhancement-mode HEMT device. It may be preferred that the enhancement-mode transistor be, for example, a silicon transistor, such as a silicon field-effect transistor (FET), which can have low blocking capability and can be provided at a low cost. The switching circuit can advantageously operate similar to, for example, a 600 volt enhancement-mode HEMT.
The enhancement-mode switching circuit may include a diode connected in parallel with the enhancement-mode transistor, such that a cathode of the diode is connected for example to both the drain of the enhancement-mode transistor and the source of the depletion-mode transistor. For example, the enhancement-mode transistor can be a silicon transistor and the diode can be the body diode of the silicon transistor.
During operation of the switching circuit, a gate-source voltage of the depletion-mode transistor will adjust so as to attempt to supply the current required by the enhancement-mode transistor, or more particularly the current required by or drawn by the diode that is in parallel with the enhancement-mode transistor. The switching circuit can easily become unstable because the depletion-mode transistor can only support a very small leakage current when it is OFF. For example, due to the diode's leakage current, the gate-source voltage of the depletion-mode transistor will adjust toward the depletion-mode transistor's pinch off voltage (i.e. its threshold voltage). As the gate-source voltage approaches or surpasses the pinch off voltage, the electric filed applied to the gate of the depletion-mode transistor can become too large, thereby rupturing the gate of the depletion-mode transistor or otherwise damaging the switching circuit. Thus, the design of the switching circuit can be complicated as the diode may leak substantial current when at a high temperature, and in some designs may be a diode that easily leaks current, such as a Schottky diode.
When the parallel enhancement-mode transistor is not used, the resulting circuit is a rectifier circuit comprising a depletion-mode transistor in series with a diode. Such rectifier circuit suffers from similar disadvantages discussed above, and the design of this conventional rectifier circuit is also complicated since the diode may leak substantial current, and in some designs may be a diode that easily leaks current, such as a Schottky diode. The conventional rectifier circuit is thus susceptible to damage by the same mechanism discussed above.
Thus, there is a need in the art for switching and rectifier circuits that can overcome the drawbacks and deficiencies in the art.